Light field imaging device

ABSTRACT

A light-field capture device including a photo-sensor, a main lens configured to refract light from a scene towards an image focal field, a micro-lens array positioned in the image focal field between the main lens and the photo-sensor, each micro-lens being configured to project a micro-lens image onto a respective dedicated area of the photo-sensor. The light-field capture device is provided with at least one compressive sensing calculation unit connected to at least one of the dedicated areas and configured to calculate data such as for example standard deviation data representative of the homogeneity of input pixels of said dedicated area, and to provide output data representative of the input pixels of said dedicated area; the output data comprising either a number of output pixels corresponding to the number of input pixels, or a single output pixel, depending on the relative value of the homoegeneity compared with a threshold.

1. TECHNICAL FIELD

The field of the disclosure relates to light-field imaging. Moreparticularly, the disclosure pertains to technologies for acquiring andprocessing light-field data.

More specifically, aspects of the invention relate to a device and amethod for acquiring output data of an image formed on the photo-sensorof a plenoptic device.

2. BACKGROUND ART

This section is intended to introduce the reader to various aspects ofart, which may be related to various aspects of the present inventionthat are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

Conventional image capture devices render a three-dimensional scene ontoa two-dimensional sensor. During operation, a conventional capturedevice captures a two-dimensional (2-D) image representing an amount oflight that reaches each point on a photo-sensor (or photo-detector)within the device. However, this 2-D image contains no information aboutthe directional distribution of the light rays that reach thephoto-sensor (may be referred to as the light-field). Depth, forexample, is lost during the acquisition. Thus, a conventional capturedevice does not store most of the information about the lightdistribution from the scene.

Light-field capture devices (also referred to as “light-field dataacquisition devices”) have been designed to measure a four-dimensional(4D) light-field of the scene by capturing the light from differentviewpoints of that scene. Thus, by measuring the amount of lighttraveling along each beam of light that intersects the photo-sensor,these devices can capture additional optical information (informationabout the directional distribution of the light rays) for providing newimaging applications by post-processing. The informationacquired/obtained by a light-field capture device is referred to as thelight-field data. Light-field capture devices are defined herein as anydevices that are capable of capturing light-field data.

Light-field data processing comprises notably, but is not limited to,generating refocused images of a scene, generating perspective views ofa scene, generating depth maps of a scene, generating extended depth offield (EDOF) images, generating stereoscopic images, and/or anycombination of these.

Among the several types of light-field capture devices disclosed inbackground art, the plenoptic devices use a micro-lens array positionedin the image focal field of the main lens, and before a photo-sensor onwhich one micro-image per micro-lens is projected. The area of thephoto-sensor under each micro-lens is, in the background art, referredto as a macropixel. Thus, the plenoptic device generates one micro-lensimage at each macropixel. In this configuration, each macropixel depictsa certain area of the captured scene and each pixel of this macropixeldepicts this certain area from the point of view of a certainsub-aperture location on the main lens exit pupil.

The raw image of the scene obtained as a result, also referred to as“output data”, is the sum of all the micro-lens images acquired fromrespective portions of the photo-sensor. These output data contain theangular information of the light field. Based on these output data, theextraction of an image of the captured scene from a certain point ofview, also called “de-multiplexing” in the following description, can beperformed by concatenating the output pixels covered by each micro-lensimage. This process can also be seen as a data conversion from the 2Draw image to the 4D light-field.

Due to the considerable amount of data generated by plenoptic devices,the compression of light field data remains an important challenge toovercome in computational photography. A few publications of backgroundart, among which U.S. Pat. No. 8,228,417B1, U.S. Pat. No. 6,476,805B1and US20090268970A1, describe various processes intended to reduce thesize of the light-field obtained after de-multiplexing the raw image.Such compression methods, even if welcomed in late stages of the imageprocessing, do not contribute in any way to reduce the size of theoutput data acquired following the capture of a scene by a plenopticdevice.

Still, the publication US20090268970A1 describes a light-fieldpreprocessing module adapted to reshape a micro-lens image by croppingit into shapes compatible with the blocking scheme of a block-basedcompression technique (e.g., squares of size 8×8, 16×16 for JPEG). Thefirst main drawback of such a method is the undifferential suppressionfor each micro-lens image of all the output pixels (and thecorresponding information) located out of the cropping perimeter. Suchlost information cannot be recovered in a later stage and shall be madeup by the implementation of a heavy resource-consuming interpolationprocess, which tend to further increase the computation load of thelight-field image processing.

The disk model of the micro-lens images depends on the intrinsicparameters of the plenoptic device, as well as the position of themicro-lens image on the photo-sensor. For peripheral parts of thesensor, the vignetting of the main lens is non-symmetric. Besides, theposition of the micro-lens images moves on the sensor by changing thezoom/focus parameter of the camera. Therefore, in the methods anddevices known of the background art, all the captured pixels (alsocalled “input pixels”) have to be stored and transferred topost-processing.

It would hence be desirable to provide a light-field capture deviceshowing improvements of the background art.

Notably, it would be desirable to provide such a device, which would beadapted to reduce the size of the raw image initially stored, whilepreserving the workable data of the light-field captured, and limitingthe computing load of the corresponding processing.

3. SUMMARY OF INVENTION

According to a first aspect of the invention there is provided, alight-field capture device is disclosed, which comprises:

-   -   a photo-sensor,    -   a main lens configured to refract light from a scene towards an        image focal field,    -   a micro-lens array positioned in the image focal field between        the main lens and the photo-sensor, each micro-lens being        configured to project a micro-lens image onto a respective area        of the photo-sensor; wherein the light-field capture device        comprises at least one calculation unit connected to at least        one of the respective areas and configured to obtain homogeneity        data representative of the homogeneity of input pixels of said        respective area, and to provide, based on the homogeneity data,        output data representative of the input pixels of said        respective area; the output data comprising either a number of        output pixels corresponding to the number of input pixels, or a        single output pixel, depending on a comparison of the        homogeneity data with a homogeneity threshold).    -   In embodiments of the invention the homogeneity data may        correspond to the standard deviation (S) of the input pixel        values

Such a light-field capture device comprises at least one compressivesensing calculation unit connected to at least one of the dedicatedareas and configured to calculate a homogeneity metric representative ofthe homogeneity of the input pixels of said dedicated area, for examplethe standard deviation of input pixels of said given area, and toprovide output data representative of the input pixels of said givenarea; the output data comprising either a number of output pixelscorresponding to the number of input pixels, or a single output pixel,depending on the relative value of the standard deviation compared witha pre-set threshold value on a homogeneity metric.

In the following description, the expression “main lens” refers to anoptical system, which receives light from a scene to capture in anobject field of the optical system, and renders the light through theimage field of the optical system. In one embodiment of the invention,this main lens only includes a single lens. In another embodiment of theinvention, the main lens comprises a set of lenses mounted one after theother to refract the light of the scene to capture in the image field.The term “given area” may be referred to as“dedicated area” refers to aspecific area of the photo-sensor, whose position on the photo-sensor isset in time. In the following description, this expression is sometimesreplaced by the expression “macropixel”. One skilled in the art willconsider them as equivalent, while keeping in mind that for the presentdisclosure, the position on the photo-sensor of such a “macropixel” isconsidered as set in time. The expression “input pixel” refers to thevalue of intensity captured/measured for each pixel of a macropixel. Theexpression “output pixel” refers to a value, which depicts the value ofintensity of at least one pixel of the macropixel. The sum of the outputpixels acquired from a macropixel forms the output data of thismacropixel. In one embodiment, the input pixels of a macropixel areseparately considered, depending on their color channel, for example inpresence of Bayer pattern on the photo-sensor or in presence of a3(three)-channel acquisition photo-sensor, to account for the colorfilter transmittances. In such an embodiment, output data are formed foreach of the color channels of the macropixel. The expression “standarddeviation” refers to a mathematical function that depends on thedisparity between the different input pixels of a macropixel. Theexpression “threshold value on a homogeneity metric” refers to areference value of the homogeneity between the different input pixels ofa macropixel.

The expression “compressive sensing calculation unit” refers to asystem, which is configured to calculate the value of the standarddeviation function and to provide output data representative of theinput pixels. Such output data comprise either one single output pixelor a number of output pixels corresponding to the number of inputpixels, based on a comparison between the standard deviation calculatedand the threshold value on the homogeneity metric. This system istherefore adapted to reduce the number of output pixels acquired to asingle one, when the image formed on the macropixel is considered ashomogenous. In this matter, the calculation of the standard deviation istargeted to determine the degree of homogeneity of the image formed onthe macropixel, known as the “captured image”, while the threshold valueon the homogeneity metric tends to set the degree of homogeneityrequired to consider the captured image as homogeneous. When thecompressive sensing calculation unit determines that the captured imagedoes not fulfill this requirement on homogeneity as set, the system doesnot proceed to the reduction in the number of output pixels, in order topreserve the diversity of the captured pixels. Thus, the workable dataand the corresponding precious information intended to be revealed infurther processing of the output data, e.g. after de-multiplexing, aresafely preserved.

In order to connect physically, e.g. by a wire, the compressive sensingcalculation unit to the macropixel to be measured, it is essential toknow, when assembling the calculation unit with the photo-sensor, theexact position of this macropixel on the photo-sensor. Since such aposition depends on the zoom/focus configuration of the light-fieldcapture device, one skilled in the art will understand that even thoughthe present invention can be embodied by light-field captured devicesimplementing or not a zoom/focus ability, in the particular case of adevice that does embody a zoom/focus ability, a specific kind of fieldlens must be added into the light-field capture device to carry out theinvention, as described in the following description.

Considering the high redundancy of light-field data, embodiments of thepresent invention allows in frequent cases a reduction in the size ofthe output data acquired from one macropixel. Moreover, considering thatthe output data acquired from a scene by a light-field capture device isformed by the sum of the output data acquired from each macropixel ofthe photo-sensor, embodiments of the invention rely on a novel andinventive approach of acquisition of light-field data by providing alight-field capture device, which is adapted to reduce significantly thesize of the raw image initially stored in the non-volatile medium, whilepreserving the workable data of the light-field captured.

The computation load of the processes implemented based on the outputdata, e.g. the de-multiplexing process, highly depends on the size ofthese output data. Therefore, the present disclosure also contributessignificantly to limit the computing load of such processes.

In one particular embodiment, the light-field capture device comprises afield lens positioned between the main lens and the micro-lens array andconfigured to set the angle of refraction of the light refracted by thefield lens.

One advantage of this technical feature is that it allows setting intime the position of the macropixels on the photo-sensor, whatever thezoom/focus configuration of the light-field capture device. One skilledin the art is then able to carry out the present disclosure with alight-field capture device that implements a zoom/focus ability.

In one particular embodiment, the field lens comprises a varifocal lens.

Such a varifocal lens is configured to adapt its object focal length inorder match the distance between the main lens and the varifocal lens.The exit pupil of the varifocal lens is then placed at infinity, whichmakes the lens image-space telecentric. In other terms, the light raysthat pass through the center of the aperture of the varifocal lens areparallel to the optical axis behind this varifocal lens. The light raysrefracted by the micro-lenses are also parallel to the optical axis andperpendicular to the photo-sensor. As a consequence, the respectivepositions of the macropixels are not only set in time, but alsoseparated one from another, when considering two adjacent macropixelsalong any direction, by a constant distance. It is therefore easier todetermine the exact position of the macropixels on the photo-sensor,prior to the assembling of the light-field capture device.

In one particular embodiment, the compressive sensing calculation unitis also configured to correct the input pixels based on pre-setcorrective weights.

One advantage of this technical feature is that the compressive sensingcalculation unit is able to correct the vignetting effects of thelight-field capture device. Indeed, such vignetting effects depend onthe technical features of the main lens and on the positions of themicro-images formed on the photo-sensor. Since such positions are set intime for the present disclosure, the corrective weights to applyrespectively to each micro-image to correct the vignetting effects canbe measured in advance, and applied to the input pixels, prior to thestoring of the output data.

In one particular embodiment, the photo-sensor is positioned at oneimage focal length from the micro-lens array.

According to this embodiment, the light-field capture device is aplenoptic camera type 1 also known as unfocused plenoptic camera. Such aplenoptic camera has the advantage to have a satisfactory angularresolution.

In one particular embodiment, the light-field capture device comprises aplurality of compressive sensing calculation units, each compressivesensing calculation unit being connected to a single dedicated area.

According to this embodiment, the complexity of the algorithm to correctthe vignetting effects is lower than in the case of a compressivesensing calculation unit handling more than one macropixel at a time.Thus, the processing unit of the compressive sensing calculation unit ismuch simpler and can be easily programmed on a FPGA board.

In one particular embodiment, at least one compressive sensingcalculation unit is individually connected to a plurality of dedicatedareas of the photo-sensor.

The advantage of such a technical feature is to put one compressivesensing calculation unit in common for a plurality of macropixels, whichallows reducing the number of calculation units. In return, thecomplexity of the algorithm that handles such data increases. Thecorresponding processing load is therefore higher and requires moreon-board memory.

In one aspect of the invention there is provided a method for acquiringoutput data from a number of input pixels, comprising:

-   -   determining homogeneity data representative of the homogeneity        of the input pixels to provide output data representative of the        input pixels, wherein    -   in the case where the homogeneity data is equal to or greater        than a homogeneity threshold, the output data comprise a number        of output pixels corresponding to the number of input pixels,    -   otherwise, in the case where the homogeneity data is less than        the homogeneity threshold, the output data comprise a single        output pixel representative of the input pixels.

One skilled in the art will understand that such a method can beimplemented on a light-field capture device according to any of theembodiments disclosed in the description.

As pointed out above for a light field capture device according to thepresent disclosure, this method allows reducing the size of the rawimage initially stored, while preserving the workable data of the lightfield captured, and limiting significantly the computing load of theprocesses implemented based on the output data acquired.

According to one embodiment, the value of the single output pixel isequal to the average value of the input pixels.

Such a technical feature allows to acquire an output pixel whose valuedepicts the best the input pixels respective values, when the standarddeviation value fulfill the homogeneity requirement set by the thresholdvalue on homogeneity metric.

According to one embodiment, the method further comprises, before thestep of calculating, of setting or modifying the threshold value for thehomogeneity metric.

It is therefore possible for an operator to adapt the degree ofhomogeneity required to consider the captured image as homogeneous, andtherefore to adapt the output data size reduction according to anyspecific need of the operator regarding the size and/or the qualityexpected on the output data.

According to one embodiment, the method further comprises, before thestep of calculating, of correcting the input pixels based on pre-setcorrective weights.

As pointed out above for a corresponding light field capture deviceaccording to the present disclosure, such a method is able to correctthe vignetting effects of the light-field capture device.

In one embodiment, the method comprises a prior step of re-initializingthe corrective weights.

Due to the fixed position of the macropixels on the photo-sensor, thevignetting effects and the corresponding corrective weights remain intheoretically unchanged, whatever the zoom/focus configuration adoptedby the light-field capture device. In practice however, theimplementation of a prior step of calibration allows adapting thecorrective weights to the minor changes that may occur on thelight-field capture device and affect the vignetting effects. As amatter of example, such minor changes may include the shift of acomponent compared to another during the assembling phase or theoperational life of the light field capture device. The replacement ofthe initial main lens by another may also account for the implementationof a new calibration step.

In one embodiment, the output data of the at least one micro-lenscomprise:

-   -   a header indicating the number of output pixels,    -   a body indicating the value of each output pixel.

The format of such output data has the advantage to be simple and easilycomputable.

According to another aspect of the invention there is provided a methodfor acquiring the output data of a scene, comprising:

-   -   Implementing, for each micro-lens of the light-field capture        device, the method for acquiring the output data of at least one        micro-lens,    -   Summing the output data respectively acquired from each        micro-lens.

A further aspect of the invention concerns a computer program product,which comprises program code instructions for implementing the methodfor acquiring the output data of at least one micro-lens of alight-field capture device, when the program is executed on a computeror a processor.

A further aspect of the invention concerns a non-transitorycomputer-readable medium comprising a computer program product recordedthereon and capable of being run by a processor, including program codeinstructions for implementing a method for acquiring the output data ofat least one micro-lens of a light-field capture device.

While not explicitly described, the present embodiments may be employedin any combination or sub-combination.

4. BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdescription and drawings, given by way of example and not limiting thescope of protection, and in which:

FIG. 1 is a schematic view of a light-field capture device according toone embodiment of the disclosure;

FIG. 2 is a schematic view of a compressive sensing calculation unit ofa light-field capture device, according to one embodiment of thedisclosure;

FIG. 3 is a flow chart of the successive steps implemented whenperforming a method for acquiring output data, according to oneembodiment of the disclosure;

FIG. 4 is an example of image captured with a plenoptic camera of thebackground art;

FIG. 5 is an example of output data acquired through the implementationof a method according to one embodiment of the disclosure; and

FIG. 6 is an example of compressive output data acquired through theimplementation of a method according to one embodiment of thedisclosure.

The components in the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the disclosure.

5. DESCRIPTION OF EMBODIMENTS

The present disclosure relates to light-field capture devices, and tomethods for acquiring output data, which embody a compressive sensingcalculation unit. Many specific details of certain embodiments of thedisclosure are set forth in the following description and in FIGS. 1 to6 to provide a thorough understanding of such embodiments. One skilledin the art, however, will understand that the present disclosure mayhave additional embodiments, or may be practiced without several of thedetails described in the following description.

Description of a Plenoptic Camera:

FIG. 1 illustrates schematically an example of light-field capturedevice: a plenoptic camera 1. Such a camera 1 comprises a main lens 2, afield lens 3, a micro-lens array 4, a photo-sensor 5 and a compressivesensing calculation unit 6.

The main lens 2 receives light from a scene to capture (not shown) in anobject field of the main lens 2 and renders the light through an imagefield of the main lens 2. In this example, the main lens 2 is embodiedby a single lens that features an optical axis X, an entrance pupilturned towards the object focal plane of the main lens 2, and an exitpupil turned toward the image focal plane. However, one skilled in theart will understand that such a main lens may be embodied by an opticalsystem comprising a set of lenses mounted one after the other to refractthe light of the scene to capture. In such kind of configuration, theentrance pupil of the main lens 2 of the following descriptioncorresponds to the one of the first lens of the optical system torefract the coming light, and the exit pupil as described corresponds tothe one of the lens that refracts this light in the image focal plane ofthe optical system.

A micro-lens array 4 is positioned in the image focal plane of the mainlens 2. In another embodiment of the disclosure, the micro-lens array 4is also positioned in the image focal field of the main lens 2, butbefore or behind the focal plane of the main lens 2. This micro-lensarray 4 includes a plurality of circular micro-lenses 7 arranged in atwo-dimensional (2D) array. In another embodiment, such micro-lenseshave different shapes, e.g. elliptical, without departing from the scopeof the disclosure. Each micro-lens 7 has the lens properties to directthe light of a corresponding micro-lens image (micro-image) 8 to adedicated area on the photo-sensor 5, referred to as macropixel 9. Thus,the plenoptic camera 1 generates one micro-image 8 at each macropixel 9.In this example, the light-field capture device described is a plenopticcamera type 1, in which all the micro-lenses 7 have the same focallength f_(μ) and in which the micro-lens array is positioned one focallength away from the photo-sensor 5. In this configuration, eachmicro-image 8 depicts a certain area of the captured scene and eachpixel of the macropixel 9 depicts this certain area from the point ofview of a certain sub-aperture location on the main lens exit pupil.Such a configuration enables maximum angular resolution and low spatialresolution to be obtained. However, one skilled in the art willunderstand that in another embodiment of the disclosure, thephoto-sensor 5 may be positioned at a distance from the micro-lens array4 non-equal to one focal length f_(μ).

In this example, the photo-sensor 5 is of a CCD (Charge-Coupled Device)type using a CMOS (Complementary Metal Oxide Semiconductor) technology.However, one skilled in the art will understand that such a photo-sensor5 may alternatively embody a neuromorphic spike based sensor (e.g.Artificial Silicon Retina) or any other type of photo-sensor known fromthe background art.

A varifocal field lens 3 is positioned in the image field of the mainlens 2, between the main lens 2 and the micro-lens array 4. The focallength of this particular field lens is adaptable to match the distanceD between the main lens 2 and the field lens 3, whatever the zoom/focusparameter of the camera. An exit pupil at infinity makes this field lensimage-space telecentric, the light rays which pass through the center ofthe exit pupil (of the field lens) being parallel to the optical axis X.Thus, both the micro-lens array 4 and the photo-sensor 5 beingrespectively arranged on a different plane perpendicular to the opticalaxis X, the position on the photo-sensor 5 of each macropixel 9 istherefore set in time, whatever the zoom/focus configuration of theplenoptic camera.

One skilled in the art will understand that in the particular case of aplenoptic camera devoid of a zoom/focus ability, the implementation of avarifocal is not necessary to carry out the disclosure, since theposition of the macropixels 9 on the photo-sensor 5 is by definition setin time.

The implementation of such a varifocal field lens 3 has severaladvantages. First, it enables an operator to know in advance theposition of the macropixel centers on the photo-sensor 5. Theimplementation of a post-capture processing step of micro-image centermapping, as described in background art, is therefore no longerrequired, which contributes to the reduction of the computing load ofthe post-capture image processing. In addition, the drops of lightintensity occurring on the edges of the main lens aperture (also called“vignetting effect”) remain invariable in time, due to the fixedposition of the macropixels 9 on the photo-sensor 5. Thus, thecorresponding corrective weights do not have to be re-estimated orapproximated before each light-field image capture. As a matter ofexample, such corrective weights are measured in a calibrationpre-capture step, and then applied to all the further captures made withthe plenoptic camera, whatever the zoom/focus configuration adopted bythe operator. While increasing the light-field capture speed, theimplementation of a varifocal field lens 3 also enables theimplementation of a vignetting correction step, prior to the acquisitionof the output data, as described in the following description.

By knowing in advance the future position of each micro-image on thephoto-sensor 5, it is possible to adapt the structure of the cameraaccordingly. In this embodiment of the disclosure, each macropixel 9 ofthe photo-sensor 5 is individually connected, e.g. by means of a wire,to a dedicated compressive sensing calculation unit 6. Thus, there areas many compressive sensing calculation units 6 as macropixels 9 on thephoto-sensor 5. In such a configuration, the complexity of the algorithmtargeted to correct the vignetting effects is low. The processing unitof such a compressive sensing calculation unit 6 is therefore notcomplex and easily programmable. One skilled in the art will understandthat in another embodiment, such a compressive sensing calculation unit6 may be shared between several macropixels 9, either in part or in itsentirety. According to this embodiment, the number of calculation unitsis reduced. In return, the compressive sensing calculation units 6 haveto handle more data and be fast enough to perform real-time calculationsall over the photo-sensor 5. In a particular embodiment of thedisclosure, the plenoptic camera comprises a single compressive sensingcalculation unit 6, which is individually connected to each of themacropixels.

FIG. 2 illustrates schematically an example of compressive sensingcalculation unit 6. Such a compressive sensing calculation unit 6comprises a microprocessor 10 a volatile memory 11, and is connectableto a non-volatile storage medium 12. The microprocessor 10 is configuredto execute program code instructions enabling implementation of at leastone part of the steps of the compressive sensing method described herebelow with reference to FIG. 3.

The non-volatile storage medium 12, e.g. a hard disk, is acomputer-readable carrier medium. The non-volatile storage medium 12enables several data to be stored, among which:

-   -   The executable program code instructions, which are executed by        the microprocessor 10 to enable implementation of the method        described here below,    -   The corrective weights that model the vignetting of the main        lens 2 for each of the micro-lenses 7,    -   A threshold value on the homogeneity metric H of each        micro-image 8,    -   The output data 13 acquired from each macropixel 9.

The volatile memory 11, e.g. a random access memory (RAM), includesregisters that enable to store temporary:

-   -   The input pixels captured for each macropixel 9,    -   The executable program code instructions,    -   The variables and parameters required for this execution.

Description of a Compressive Sensing Method for Acquiring Output Data:

FIG. 3 illustrates in more detail the successive steps implemented bythe method for acquiring the output data 13 of at least one micro-lens 7of the light-field capture device 1 according to one embodiment of thedisclosure.

Following the initialization step 14, the program runs the step INPUT 15that offers to the operator the possibility to re-initialize the valuesof:

-   -   The corrective weights of each of the micro-lenses 7,    -   A threshold value on the homogeneity metric of each micro-image.

In another embodiment of the disclosure, these data are pre-set in thenon-volatile storage medium 12 and the program only allowsre-initializing them upon request.

The program code instructions, the corrective weights, and the thresholdvalue on the homogeneity metric H are then transferred from thenon-volatile storage medium 12 to the volatile memory 11 so as to beexecuted by the microprocessor 10.

Following the step INPUT 15, the program offers the operator thepossibility of initiating the step CAPTURE 16, which consists in savingon the volatile memory 11 of each compressive sensing calculation unit 6the input pixels of the corresponding macropixel 9.

Following the step CAPTURE 16, the program runs the step CORRECTION 17,which consists in correcting the values of the input pixels capturedbased on the corrective weights pre-stored in the volatile memory 11.

Following the step CORRECTION 17, the program runs the step CALCULATION18, which comprises obtaining homogeneity data representative of thehomogenity of the input pixels, by for example calculating the standarddeviation S of the input pixels.

In this example, the standard deviation S may be expressed as:

$S = \sqrt{\frac{\sum_{N}\left( {d - d^{s}} \right)^{2}}{N}}$

Wherein N is the number of input pixels, d is the value of each inputpixel, and d^(S) is the average value of the input pixels.

The calculated standard deviation S depicts the homogeneity of the scenecaptured by the targeted macropixel 9 and the corresponding micro-lens7.

In another embodiment, this standard deviation S may be calculated basedon a slightly different algorithm, which could potentially bereprogrammable by an operator, when running the step INPUT 15.

In one particular embodiment, a plurality of standard deviation values Sare separately estimated on every color channel of a macropixel 9, forexample in presence of Bayer pattern on the photo-sensor 5 or inpresence of a 3(three)-channel acquisition photo-sensor, to account forthe color filter transmittances. Therefore, for every macropixel 9,three standard deviations S are calculated. Each is then thresholdedaccording to a channel dependent threshold value on a homogeneity metricH. The thresholded values are then summed to decide whether themacropixel 9 contains high homogeneity in at least one channel.

The value of the calculated standard deviation S is compared to thethreshold value on the homogeneity metric H pre-stored in the volatilememory 11.

If (step 19) the standard deviation S is equal to or greater than thethreshold value H, all the corrected input pixels are saved as outputpixels (step 20) on the non-volatile storage medium 12, forming theoutput data of the targeted macropixel 9.

If (step 19) the standard deviation S is lower than the threshold valueH, only one pixel value is saved as output pixel (step 21) on thenon-volatile storage medium 12, thereby forming the output data of thetargeted macropixel 9. The value of this single output pixel is equal tothe average value of the input pixels of the targeted macropixel 9.

The output data acquired by the plenoptic camera 1 are then the sum ofthe output data acquired from each macropixel 9 of the photo-sensor 5.

In this example, the output data 13 acquired comprise both a headerindicating the number of output pixels, and a body indicating the valueof the output pixels. This image format has the advantage of beingsimple and easily computable. One skilled in the art will understandthat this image format may be replaced in another embodiment of thedisclosure by another image format, known from the background art.

The advantage of the method is to reduce the redundancy of thelight-field data, and therefore the size of the output data acquired,while efficiently representing the information captured for eachmacropixel 9.

In another embodiment of the disclosure, the program runs a step ofcalibration, prior to the step CAPTURE 16. This additional step aims tore-initialize the vignetting corrective weights corresponding to each ofthe micro-lenses 7. For this step, the threshold value H is set to “0”and the corrective weights are all initialized by “1”. An image of awhite diffuser is captured. Since each micro-lens captures thevignetting effect occurring at the edges of the main lens 2, theestimated homogeneity of each micro-lens is greater than the threshold(set to 0). Therefore, all of the input pixels are saved as outputpixels. The acquired output data provide the intensity modulations thatare introduced by the vignetting of the main lens 2. Therefore, thefinal corrective weights can be set to the inverse of the capturedpixels of each macropixel 9 and stored in the volatile memory 11 of thecorresponding compressive sensing calculation unit 6.

One example on experimental data is shown in FIGS. 4, 5 and 6. Thestandard deviations D of the micro-lens images 8 are calculated andthresholded to determine the number of micro-lens images that depicthomogeneous areas of the scene as captured without compressive sensingimplementation (as illustrated by FIG. 4).

In FIG. 5, the micro-lens images that contain high frequency informationof the scene are illustrated with a white point. Each macropixel isdepicted using a single pixel. The corresponding measurement shows thatfor a threshold value H of 10% of the pixel range, the percentage ofnon-uniform micro-lens image is equal to 43,0675%, while retaininginformation. Thus, we only require saving and transmitting 50% of thecaptured data to provide the same information, the rest being redundant.The size of the output data is therefore half the size of the capturedinput data.

The reduction in size of the output data can be increased as desired.Following the same scheme, the value of the threshold H is increased to50% of the pixel range to obtain good quality output data (illustratedin FIG. 6) of reduced size, with a percentage of non-uniform micro-lensimage equal to 23,4409%. In this example, the file size is reduced toone fourth.

As will be appreciated by one skilled in the art, aspects of the presentprinciples can be embodied as a system, method or computer readablemedium. Accordingly, aspects of the present principles can take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, and so forth), or anembodiment combining software and hardware aspects.

When the present principles are implemented by one or several hardwarecomponents, it can be noted that an hardware component comprises aprocessor that is an integrated circuit such as a central processingunit, and/or a microprocessor, and/or an Application-specific integratedcircuit (ASIC), and/or an Application-specific instruction-set processor(ASIP), and/or a graphics processing unit (GPU), and/or a physicsprocessing unit (PPU), and/or a digital signal processor (DSP), and/oran image processor, and/or a coprocessor, and/or a floating-point unit,and/or a network processor, and/or an audio processor, and/or amulti-core processor. Moreover, the hardware component can also comprisea baseband processor (comprising for example memory units, and afirmware) and/or radio electronic circuits (that can comprise antennas),which receive or transmit radio signals. In one embodiment, the hardwarecomponent is compliant with one or more standards such as ISO/IEC18092/ECMA-340, ISO/IEC 21481/ECMA-352, GSMA, StoLPaN, ETSI/SCP (SmartCard Platform), GlobalPlatform (i.e. a secure element). In a variant,the hardware component is a Radio-frequency identification (RFID) tag.In one embodiment, a hardware component comprises circuits that enableBluetooth communications, and/or Wi-fi communications, and/or Zigbeecommunications, and/or USB communications and/or Firewire communicationsand/or NFC (for Near Field) communications.

Furthermore, aspects of the present principles can take the form of acomputer readable storage medium. Any combination of one or morecomputer readable storage medium(s) may be utilized.

Thus for example, it will be appreciated that any flow charts, flowdiagrams, state transition diagrams, pseudo code, and the like representvarious processes which may be substantially represented in computerreadable storage media and so executed by a computer or a processor,whether or not such computer or processor is explicitly shown.

Although the present disclosure has been described with reference to oneor more examples, workers skilled in the art will recognize that changesmay be made in form and detail without departing from the scope of thedisclosure and/or the appended claims.

1. A light-field capture device comprising: a photo-sensor, a main lensconfigured to refract light from a scene towards an image focal field, amicro-lens array positioned in the image focal field between the mainlens and the photo-sensor, each micro-lens being configured to project amicro-lens image onto a respective area of the photo-sensor; wherein thelight-field capture device comprises at least one calculation unitconnected to at least one of the respective areas and configured toobtain homogeneity data representative of the homogeneity of inputpixels of said respective area, and to provide, based on the homogeneitydata, output data representative of the input pixels of said respectivearea; the output data comprising either a number of output pixelscorresponding to the number of input pixels, or a single output pixel,depending on a comparison of the homogeneity data with a homogeneitythreshold.
 2. A light field capture device according to claim 1, whereinthe homogeneity data corresponds to the standard deviation of the inputpixels.
 3. A light-field capture device according to claim 1, comprisinga field lens positioned between the main lens and the micro-lens arrayand configured to set the angle of refraction of the light refracted bythe field lens.
 4. A light-field capture device according to claim 3,wherein the field lens comprises a varifocal lens.
 5. A light fieldcapture device according to claim 1, wherein the at least onecompressive sensing calculation unit is further configured to correctthe vignetting effects of the light-field capture device.
 6. A lightfield capture device according to claim 1, wherein the photo-sensor ispositioned at one image focal length (f_(μ)) from the micro-lens array.7. A light-field capture device according to claim 1, comprising aplurality of compressive sensing calculation units, wherein eachcompressive sensing calculation unit is connected to a single dedicatedarea.
 8. A light field capture device according to claim 1, wherein atleast one compressive sensing calculation unit is individually connectedto a plurality of dedicated areas of the photo-sensor.
 9. Method foracquiring output data from a number of input pixels, comprising:obtaining homogeneity data representative of the homogeneity of theinput pixels to provide output data representative of the input pixels,wherein in the case where the homogeneity data is equal to or greaterthan a threshold, the output data comprise a number of output pixelscorresponding to the number of input pixels, otherwise, in the casewhere the homogeneity data is less than the threshold, the output datacomprise a single output pixel representative of the input pixels. 10.Method according to claim 9 wherein obtaining the homogeneity datacomprises calculating the standard deviation of the input pixels. 11.Method according to claim 9, wherein the value of the single outputpixel is representative of the average value of the input pixels. 12.Method according to claim 9, further comprising, before obtaining thehomogeneity data, correcting (the input pixels based on pre-setcorrective weights.
 13. Method according to claim 9, wherein the outputdata comprise: a header indicating the number of output pixels, a bodyindicating the value of each output pixel.
 14. Computer program product,comprising program code instructions for implementing the methodaccording to claim 9, when the program is executed on a computer or aprocessor.
 15. A non-transitory computer-readable medium comprising acomputer program product recorded thereon and capable of being run by aprocessor, including program code instructions for implementing a methodaccording to claim 9.